1. Field of the Invention
This invention relates generally to magnetic resonance imaging (MRI) systems. In particular, it relates to the spatial arrangement of a pair of columns in a two-column magnetic circuit of the main static magnetic field generator to permit open and unobstructed access to the image volume along a direction perpendicular to the longitudinal axis of the patient and patient transport mechanism.
2. Related Art
The art of magnetic resonance imaging (MRI) is now well developed and several different types of MRI systems are commercially available. In all of them, some static magnetic field generator is provided to produce a very strong static magnetic field H.sub.o (controlled pulses of spatial gradients along three mutually orthogonal coordinate axes are typically also generated by related structures). The static magnetic field is typically approximately homogenous within a predefined imaging volume (e.g., a spherical volume 30-50 cm in diameter) and the controlled gradients are typically approximately linear with respect to spatial displacements therewithin.
In conjunction with coordinated magnetic gradient pulses, a programmed sequence of radio frequency pulses is transmitted into body portions located within the imaging volume at predetermined frequency distributions so as to selectively nutate the magnetic moment of certain atoms by predetermined amounts in accordance with well-known nuclear magnetic resonance (NMR) principles. After cessation of such transmitted RF pulses, the NMR nutated atoms tend to relax back toward alignment with the static magnetic field H.sub.o and, in the process, produce characteristic NMR RF signals. Such RF signals are received (typically under the influence of a magnetic gradient pulse), detected and processed to thereafter produce a desired MRI image of the body portion located within the imaging area in accordance with any one of many known MRI techniques as will be appreciated by those in the art. As noted, the transmitted RF pulses are synchronized with a special sequence of current pulses passed through various magnetic gradient coils during the imaging process so as to effect spatial information encoding processes and/or to provide known types of NMR phasing control.
In some MRI apparatus, the static magnetic field H.sub.o and/or the magnetic gradient coils are realized in the form of large solenoidal coils or, in the case of Gx, Gy, Gz gradient coils, saddle-shaped coils conformed to a generally tubular configuration. In such cases, it is naturally necessary for patient access to the imaging volume to be provided only along a narrow tunnel through the tubular shaped apparatus. With some patients, this may give rise to claustrophobic reactions. It also makes it extremely cumbersome to access the image volume (e.g., so as to adjust the relative positioning of RF transmit and/or receive coils, to perform interventional procedures on the relevant patient anatomy or to attend to other patient needs).
Other types of MRI systems utilize a pair of magnetic poles (e.g., permanent magnets or electromagnets with ferromagnetic or air cores--possibly with super-conducting windings) disposed on opposite sides of the image volume to create the requisite static magnetic field H.sub.o. Necessary magnetic circuits for return flux (i.e., outside the image volume) between the magnetic poles have been proposed in various forms. Presently available commercial devices typical use four columns disposed at the four corners of the main magnet (e.g., see U.S. Pat. No. 4,829,252--Kaufman). Two-column main magnet MRI systems have also been demonstrated and described (e.g., see U.S. Pat. No. 4,943,774--Breneman et al). The entire contents of U.S. Pat. Nos. 4,829,252 and 4,943,774 are hereby incorporated by reference.
As described in U.S. Pat. No. 4,829,252--Kaufman, the earliest use of four-column magnetic circuits did not take any advantage of its potential for open access. Instead, early use of such four-column structures impeded access by various means--except along a narrow patient tunnel aligned with the longitudinal axis of a patient transport mechanism. Accordingly, such early four-column systems were subject to all of the usual claustrophobic patient reactions. They also substantially prevented patient assistance and/or active interventional procedures during what can be a fairly lengthy MRI procedure. The Kaufman '252 patent describes and claims an invention for taking advantage of the four-column magnetic circuit so as to leave the final MRI system with open transverse access from the sides to the patient image volume.
U.S. Pat. No. 4,943,774 is representative of prior art proposals for using only two-columns in the flux return portion of the main magnet magnetic circuit. As described in the '774 patent and as recently demonstrated by others at an industry symposium, the symmetry axis of the two-column magnetic circuit was disposed perpendicular to the longitudinal axis of the patient and patient transport mechanism. The diameter of each return flux column is approximately the same as the diameter of the image volume. Accordingly, even though the two-column structure theoretically presents more overall openness to the image volume, the prior art utilization of such two-column structures has actually resulted in essentially no access to the image volume from directions that are directly perpendicular to the longitudinal axis of the patient and patient transport mechanism. That is, prior art usages of such two-column structures have placed the two columns of the magnetic circuit so as to directly obstruct access to the image volume along a direction perpendicular to the longitudinal patient axis. Presumably such system designs were limited in this fashion out of unfounded fear or concerns (e.g., that any other spatial location of the yoke structures might adversely impact the desired homogeneity of magnetic field within the image volume).